Nowadays gas sensors are used in many fields of technology in order to satisfy increasing demands placed on environmental compatibility and safety. One example of an application for gas sensors consists of detecting ammonia (NH3) in the exhaust gas system of diesel engines. This may occur during selective catalytic reduction, or SCR, which is used to reduce the emission of nitrogen oxides (NOx).
It is known to use ‘mixed potential sensors’ to detect ammonia in car exhaust gas systems. These electrochemical gas sensors have a first and a second electrode, the second electrode being made of a different material to the first electrode, which is in contact with the gas environment in which, for example, ammonia is to be detected. The two electrodes are connected via an ion-conducting, i.e. electrolytic material, for example, YSZ (yttrium-stabilized zirconium oxide). Depending on the gas concentration, an electromotive force (EMF), i.e. a voltage, is set between the electrodes. This is used as a measuring signal. In this case the second electrode is normally referred to as a passive electrode and is made, for example, of platinum. The first electrode is normally referred to as an active electrode and consists of a complex mixture of a metal oxide, for example bismuth vanadium oxide (BiVO4), with an admixture of, for example, a metal such as 5% magnesium. In this case the complex material mixtures must satisfy a range of requirements. They must therefore be sufficiently electrically conductive. They must also be sufficiently thermally and chemically stable for the gas environments, which are often hot and aggressive, in which they will be used. Lastly, the complex material mixtures must be as catalytically selective as possible, i.e. they must promote as few chemical reactions as possible, ideally just one, in the gas environment to be measured. A drawback of known sensors is that their construction, in particular the construction of the complex first electrode, is very involved.